1. Field of the Invention
This invention relates to a synchronous signal separation circuit for a television set and the like, and more particularly to a synchronous separation circuit incorporating C-MOS inverters as an active element.
2. Description of the Prior Art
Recently, it has been able to make a television set compact and even a portable liquid crystal television has been developed. A conventional television set using a CRT has incorporated a bipolar transistor as an active element in a synchronous separation circuit.
However, in a compact and portable electronic equipment such as a portable liquid crystal television and the like which deals with information, it is preferable to use a C-MOS LSI for processing information, so as to make parts compact and reduce the number of parts as well as reduce current consumption, wherein a synchronous signal is extracted from a synthetic video signal, which is an analog signal, by a C-MOS circuit to digitize the signal. To this end, there has arisen necessity of similarly making a synchronous separation circuit of C-MOS. In such a circumstance as above, the present invention relates to an improvement of the synchronous separation circuit with C-MOS inverters.
FIGS. 1 and 2 show a conventional synchronous separation circuit using a C-MOS inverter. Namely, FIG. 1 shows an example of asynchronous separation circuit using a C-MOS inverter as an active element. FIG. 2A shows a waveform at an input terminal of the C-MOS inverter upn receiving a synthetic video signal. FIG. 2B shows a graph of an outputted synchronous signal. In FIG. 1, C1 designates a capacitor of about 0.1 .mu.F for blocking direct current relative to a synthetic video signal; I1 a C-MOS inverter for amplifying a synchronous signal in the synthetic video signal; R1 a resistor of about 5 M.OMEGA.; R2 a resistor of about 1 M.OMEGA.; R3 a resistor of about 10 K.OMEGA.; C2 a smoothing capacitor of about 100 pF; and D1 a diode.
In FIG. 2A, an axis of ordinates indicates a potential wherein a reference voltage VF is a change-over potential of the C-MOS inverter, while an axis of abscissas corresponds to a time. The graph shows a waveform of an input to the C-MOS inverter, that is, a waveform of the sum of the synthetic video signal and a d.c. bias voltage given by the resistors R1 to R3, capacitors C1 to C2, and diode D1.
FIG. 2B shows a waveform of an output when the synthetic video signal is inputted to the circuit of FIG. 1, wherein an axis of abscissas corresponds to a time, which is in accordance with the axis of abscissas of FIG. 2A. The resistor R1 serves to equalize a potential at an input terminal of the C-MOS inverter to a potential at an output terminal thereof, while a circuit constituted of the resistors R2 and R3, capacitor C1 and diode D1 serves to lower a potential at the input terminal of the C-MOS inverter than a potential at the output terminal thereof by almost an amount of voltage drop of the diode D1. When the synthetic video signal is inputted to the circuit of FIG. 1, the above-mentioned two functions reach equilibrium, and as shown in FIG. 2A, a lowermost potential VL1 of the synchronous signal in the synthetic video signals is fixed at such a position as to be lowered from the change-over potential VF of the C-MOS inverter I1 to substantially half amount of the voltage drop of the diode. As a result, the synchronous separation signal as shown in FIG. 2B is obtained.
However, according to the circuit as shown in FIG. 1, an output voltage of the C-MOS inverter I1 is always varied, and therefore it is necessary to make a circuit having a long time constant so as to obtain a stable d.c. bias. To meet the requirements, it is obliged to enlarge a capacitance of the d.c. blocking capacitor C1. Accordingly, a volume of the capacitor C1 must be enlarged, thereby causing interference with making of a compact electronic equipment and reduction of cost.